added unit testing, and started implementing unit tests...phew

node_modules/union-value/index.js

@@ -0,0 +1,30 @@
+'use strict';
+
+var isObject = require('is-extendable');
+var union = require('arr-union');
+var get = require('get-value');
+var set = require('set-value');
+
+module.exports = function unionValue(obj, prop, value) {
+  if (!isObject(obj)) {
+    throw new TypeError('union-value expects the first argument to be an object.');
+  }
+
+  if (typeof prop !== 'string') {
+    throw new TypeError('union-value expects `prop` to be a string.');
+  }
+
+  var arr = arrayify(get(obj, prop));
+  set(obj, prop, union(arr, arrayify(value)));
+  return obj;
+};
+
+function arrayify(val) {
+  if (val === null || typeof val === 'undefined') {
+    return [];
+  }
+  if (Array.isArray(val)) {
+    return val;
+  }
+  return [val];
+}